The research proposed here seeks to determine the electronic structures of some iron containing enzymes using Mossbauer spectroscopy and electron paramagnetic resonance. The experiments include: 1) Further studies of the cluster structures and reaction mechanism of aconitase, a key enzyme in the citric acid cycle. 2) Continuation of research into the nature of the active sites of dioxygenases. These enzymes cleave catechols by oxygen insertion and are of major importance in the biodegradation of aromatic compounds in the biosphere. 3) Continuation of Mossbauer studies of cytochrome oxidase, the terminal enzyme of the respiratory chain. The a3-site of the enzyme, the locus of oxygen reduction, will be studied with enzymes isolated from beef heart mitochondria, Thermus thermophilus and Nitrosomonas europeae. 4) It is also proposed to study a lignin-degrading enzyme from the fungus P. chrysosporium.